I find this area of Parish Council work quite interesting. I agree that only the clerk (at the behest of council) should reply, providing that reply 'for and on behalf of council'. The clerk has delegated authority to reply on behalf of council on many matters, of course. IMHO any councillor can write to anyone they like (just as they can talk to them) but you have to clearly show its from you as an individual and not as a representative of the council.